1. Field of the Invention
The present invention generally relates to a shrouded turbine blade for a gas turbine engine and more specifically to an improved airfoil profile resulting in improved creep rupture life.
2. Description of Related Art
A gas turbine engine typically comprises a multi-stage compressor, which compresses air drawn into the engine to a higher pressure and temperature. A majority of this air passes to the combustors, which mix the compressed heated air with fuel and contain the resulting reaction that generates the hot combustion gases. These gases then pass through a multi-stage turbine which drives the compressor, before exiting the engine. A portion of the compressed air from the compressor bypasses the combustors and is used to cool the turbine blades and vanes that are continuously exposed to the hot gases of the combustors. In land-based gas turbines, the turbine is also coupled to a generator for generating electricity.
In order to reduce operational costs of the gas turbine engine, it is desirable to increase the component life of replaceable items, such as turbine blades and vanes. This demand for more durable turbine components continues to increase today, especially with higher operating temperatures that are required to increase turbine engine performance. Although these components can often be repaired multiple times, eventually they must be replaced. In order to accommodate higher temperatures and longer exposure to such temperatures, enhanced turbine blade configurations are required, either through more efficient cooling or reduced stress of the turbine blade airfoil.
A known life-limiting factor for a turbine blade is creep, which is plastic deformation of the blade caused by slippage along crystallographic directions in the individual crystals of the metallic blade. This typically occurs when a turbine blade is exposed to high operating temperatures and a high stress level for an extended period of time, causing the blade to stretch. Depending on the operating conditions, the turbine blade will eventually stretch or creep to a shape that is no longer within the manufacturers acceptable limits and must be replaced. Creep life for turbine blades can be measured in terms of useable creep life, that is the amount of time in hours, until the part must be replaced, or in terms of creep rupture life, that is the amount of time until failure or rupture of the material. One skilled in the art of turbine blade design will understand that the useable creep life of a turbine blade is a percentage of the creep rupture life. However, this useable creep life varies amongst turbine blade manufacturers.
In order to extend the creep life of a turbine blade, either the temperature that the blade is exposed to must be reduced through enhanced cooling, or the stress on the blade must be reduced. The stress level on the blade can be reduced by altering the aerodynamic shape of the airfoil portion of the blade, which in turn, reduces the mechanical load. However, when attempting to enhance cooling, given a known amount of cooling air, new and more costly casting and manufacturing processes may be necessary in order to improve the cooling.
Therefore, what is desired is a turbine blade that utilizes the existing cooling air, yet has an airfoil profile that has increased life due to a lower airfoil creep rate as a result of lower mechanical loading.